Task/Issue URL: https://app.asana.com/1/137249556945/task/1215234100141222?focus=true

Description

When a user performs a search while Duck.ai is disabled, route the query to
noai.duckduckgo.com instead of duckduckgo.com so the server can serve an AI-free
SERP. Gated behind a new remote feature flag.

Rather than change AppUrl.Url.HOST (referenced in ~20 places — cookies, URL
classification, JS-messaging allowlists, sync, surveys), the host swap is scoped to
search-query URL construction only. A new DuckDuckGoSerpHostProvider owns the
decision (noAiSerpHost flag enabled and !duckChat.isEnabled()) and the host
string, and is consulted at the query-building sites. Because DuckDuckGoUrlDetector
matches on the registrable domain (topPrivateDomain()), noai.duckduckgo.com is
still recognised as a DDG URL everywhere downstream, so the other HOST usages are
untouched.

Details:

• New flag androidBrowserConfig.noAiSerpHost (default INTERNAL during rollout).
• New AppUrl.Url.NO_AI_HOST = "noai.duckduckgo.com".
• DuckDuckGoSerpHostProvider reads the flag + Duck.ai state fresh per query (no caching),
  so it reflects the user toggling AI mid-session.
• Wired into QueryUrlConverter (omnibar / navigated queries) and the AutoComplete
  visited-SERP suggestion URL.
• When the noai host is used, the existing kbg=-1 (HIDE_DUCK_AI) param is no longer
  appended in DuckDuckGoRequestRewriter — the host itself signals AI-off, so the two
  are mutually exclusive.

Steps to test this PR

Use an internal build (./gradlew installInternalDebug) — the flag defaults on there.

Queries route to noai when AI is disabled

• Turn Duck.ai off (Duck.ai settings).
• Search from the address bar; confirm the SERP loads from noai.duckduckgo.com
  (check the loaded URL via proxy/web inspector) and that kbg=-1 is NOT in the query.
• Tap a previous search suggestion in autocomplete → also routes to noai.duckduckgo.com.

Queries stay on duckduckgo.com when AI is enabled

• Re-enable Duck.ai; search again → SERP loads from duckduckgo.com.

Flag off = unchanged behaviour

• Disable the noAiSerpHost flag (feature-flag inventory / dev settings). With Duck.ai
  off, searches go to duckduckgo.com and kbg=-1 is present, exactly as before.

Typed URLs unaffected

• With AI off + flag on, type a full URL (e.g. example.com) → navigates there unchanged
  (no host rewrite).

UI changes

No UI changes — the AI-free SERP is rendered server-side by the noai host.